1. Field of the Invention
This invention relates to a sensor for detecting the number of revolutions of a rotating member and to a motor-driven actuator provided with the sensor.
2. Description of the Related Art
Sensors of this type are used, for example, to count the number of revolutions of a motor which is driven continuously. Specifically, this type sensor comprises, as shown in FIG. 13, a single magnet B secured to the outer peripheral surface of a rotary shaft A rotated by a motor, and a magnetic probe C arranged near the outer peripheral surface of the rotary shaft A. The magnetic probe C is composed of a Hall IC including a Hall element, for example, and is positioned such that, as the rotary shaft A rotates, the magnet B passes just under the probe C.
The Hall element of the magnetic probe C outputs an analog voltage signal D, as shown in FIG. 14, upon detecting the magnetism of the magnet B each time the magnet B passes thereby. The voltage signal D is converted into a digital pulse signal within the magnetic probe C. Accordingly, pulse signals E corresponding in number to passages of the magnet B are output from the magnetic probe C. In FIG. 14, two lines parallel to the horizontal axis indicate an H slice level and an L slice level respectively defining the positive going portion and negative going portion of the pulse signal E.
Therefore, by counting the pulse signals E from the magnetic probe C, it is possible to obtain the number of revolutions of the rotary shaft A, i.e., the motor.
The above-mentioned sensor produces pulse signals E corresponding in number to the revolutions of the rotary shaft A while the shaft A is rotated at a predetermined speed. However, in the case in which the power supply to the motor is stopped and thus the rotary shaft A is stopped after being rotated for a time by inertia such that the magnetic probe C and the magnet B are situated relative to each other as shown in FIG. 13, if the rotary shaft A is vibrated or there is looseness in the assembly of the rotary shaft A, voltage signals are continuously output from the Hall element of the magnetic probe C, as indicated at Dc in FIG. 14. Accordingly, the pulse signals are also continuously output from the magnetic probe C, as indicated at Ec. As a result, so-called chattering occurs in the sensor output signal, making it impossible to accurately detect the number of revolutions of the rotary shaft A.
This disadvantage arises from the fact that the use of a single magnet B secured to the rotary shaft A inevitably brings about a small hysteresis angle .theta.1 (about 5.degree.), taken in terms of the rotational angle of the rotary shaft A, which is defined by the H and L slice levels in relation to the waveform of the voltage signal D shown in FIG. 14. When the rotary shaft A, i.e., the magnet B, is vibrated over the hysteresis angle .theta.1, the aforesaid disadvantage occurs.
To eliminate the erroneous counting of revolutions caused by the above chattering, those pulse signals E whose pulse interval t1 is improbably short may be neglected. Namely, provided that the region occupied by the magnet B is 20.degree. and the other region is 340.degree. in terms of the rotational angle of the rotary shaft A, and that the interval at which the magnetic probe C detects the magnetism of the magnet B, i.e., the interval between H (or L) level signals, is 7 ms at the shortest taking account of the rotational speed of the motor, those signals Ec whose pulse interval is shorter than 7 ms may be filtered out and thus neglected. In this case, to measure the interval t1, the H level period t2 must be actually detected. However, the region of the magnet B in terms of the rotational angle of the rotary shaft A is as small as 20.degree. as mentioned above, and accordingly, the H (or L) level period t2 is very short. It is therefore difficult to detect the H (or L) level period t2 and accordingly to filter out the pulse signals Ec.
Even if the pulse signals Ec can be filtered out in the aforesaid manner, the filtering does not effectively function in the case in which the rotary shaft A is slowly vibrated when the magnet B and the magnetic probe C are located relative to each other as shown in FIG. 13 with the rotary shaft A stopped. In such a case, chattering of the pulse signals cannot be eliminated and the number of revolutions of the rotary shaft A cannot be counted accurately.
This disadvantage is particularly serious when the above sensor for detecting the number of revolutions is used with a motor-driven actuator, wherein the rotary shaft A is rotated forward and reversely by a motor which is driven continuously, and the forward and reverse rotations of the rotary shaft A are converted, for example, to reciprocating motions of the valve member of a valve, i.e., opening and closing motions of the valve, to thereby obtain the valve opening of the valve member based on the number of revolutions of the rotary shaft A. In this case, if chattering occurs in the pulse signals from the sensor, the number of revolutions of the rotary shaft A cannot be counted accurately, and accordingly, the opening of the valve member cannot be controlled with high accuracy.